Modern internal combustion engines used in vehicles are generally provided with at least one camshaft. The camshaft cooperates with cam lobes of intake and exhaust valves of cylinders of the engine such that a rotation of the camshaft opens and closes the valves. The camshaft is generally driven by the crankshaft of the engine, wherein a rotation of the crankshaft is transmitted to the camshaft by cam belt or cam chain engaged with a sprocket connected to the camshaft.
To achieve at least one of the benefits of: a lower fuel consumption; increased power, or lower emissions of the engine, a rotational phase difference between the crankshaft and the camshaft is regulated as a function of a plurality of parameters, e.g. the temperature of the engine. To regulate the phasing, the prior art teaches, inter alia, the use of a spline VVT (Variable Valve Timing). Typically, a spline VVT has an outer wheel attached to the sprocket, an inner wheel attached to the camshaft and a center wheel located in-between, meshing with both of the outer and inner wheels. Generally, the outer wheel is inwardly provided with a helical spline and the inner wheel is outwardly provided with a helical spline with an opposite groove direction. The center wheel is provided with inward and outward splines, corresponding to the splines of the inner and outer wheels.
When a change in the rotational phase between the crankshaft and the camshaft is requested, the center wheel is displaced axially, resulting in a rotation of the inner wheel with respect to the outer wheel due to the interaction of the splines of the outer, center and inner wheels. Hence, the camshaft is rotated with respect to the sprocket resulting in a phase lag or lead with respect to the crankshaft.
Prior art teaches various ways of imparting the axial displacement on the center wheel. For example, previously known solutions utilize hydraulic arrangements for applying a hydraulic pressure on either side of a piston fixed to the center wheel to impart an axial motion. However, this generally results in a complex hydraulic system several components of which are rotating with the spline VVT when the engine is running.
Prior art, e.g. WO 2006/025173, also teaches that a permanent-magnet rotary drum may be attached onto the center wheel. The center wheel may be displaced by braking or accelerating the drum by an electromagnetic clutch fixedly connected to the engine. However, the aforementioned solution requires that the rotary drum is imparted the same rotational velocity as the center wheel to maintain a selected phase difference between the rotation of the camshaft and the rotation of the crankshaft. This may require a power supply to the spline VVT system whenever the engine is running.